1. Field of the Invention
This invention relates to methods and apparatuses for the control of sand or formation particles in a wellbore.
2. Description of the Prior Art
For a hundred years attempts have been made to develop a method to keep sand out of a well and out of the fluids produced from a well. In California and the Gulf Coast of Louisiana and Texas sand problems are common. Control of sand production is a necessity. Uncontrolled sand infiltration is the primary cause for a major percentage of repairs necessary to maintain or improve production from wells. Formation sand causes tubing and perforations to plug. The sand also erodes and damages wellhead and production equipment. Sand movement occurs in almost all geologic formations including the shallow formations of the Tertiary age and in depths above 12,000 feet.
A variety of theories have been proposed to explain sand problems, including (a) higher flow rates and higher fluid viscosity increase the drag forces of flowing fluid; (b) production due to the dissolution of cementing materials causes a reduction in formation "strength" or a reduction in capillary forces with increasing water saturation, and (c) increased saturation causes reduced relative permeability to oil, thereby increasing pressure drawdown for a given production rate.
The various conventional sand control methods require the pumping of fluids to the bottom of the hole. These fluids and their components can damage the formation. The primary cause for formation damage is contact of the formation with a foreign fluid. This fluid could be a drilling mud, completion or workover fluid, or the reservoir fluid if its basic characteristics are altered. The damage itself results from the solids content of the foreign fluid or a chemical reaction between fluid and some component of the reservoir.
Solids plugging of the producing formation is common. Solids could include the solids employed with various conventional sand control systems. Small solids can cause severe plugging since they can be conveyed a considerable distance into the formation.
Diagnosis and remedial procedures are difficult. Diagnosis procedures include injection or productivity tests to determine the "skin effect," production log surveys defining intervals not contributing to the total flow, or comparison to comparable intervals in offset wells. Remedial measures include acidizing to remove swollen clays and oil-treating chemicals to break emulsions or reverse previous wettability changes.
Conventional sand control techniques fall into two categories: gravel packing and plasticizing. The two major types of gravel packing are conventional gravel packing and washdown gravel packing.
In conventional gravel packing a working string is introduced into the wellbore with a gravel pack screen at the bottom of the string. A shorter screen called a "tattletale" is placed above the gravel pack screen. The chosen gravel pack sand is pumped down the working string to surround the screen and pack the annulus between string and wellbore in the area of the perforations which extend into the producing formation. Once the packing has reached a level above the tattletale, pumping is stopped. Then prior to commencing production, the working string must be removed from the wellbore and a production string must be inserted. In a typical operation fluid is pumped at 2 to 4 barrels per minute and the fluid is composed of about half a pound of packing sand for each gallon of pumping fluid (usually water). Typically with this type of operation it will take about twenty-four hours to pack 100 feet of casing at a depth of 5,000 feet.
Conventional gravel packing has serious disadvantages. Excessive amounts of water are lost into the formation causing further damage and inhibiting production. Unwanted particles from the packing fluid enter the formation causing further damage and production inhibition. Water pumped past the perforation openings creates currents and vortices which result in the unwanted intermixing of formation particles and packing sand.
In wash down gravel packing the wellbore is filled with a desired amount of gravel pack sand, usually at least to a level above the perforations. The tubing string used to pump the packing is removed and a production string having a gravel pack screen at the bottom and a wash pipe therein is inserted into the packing. Water flowing down the wash pipe washes the gravel pack sand away to permit the screen and string to descend to a desired depth in the packing. This is necessary because the gravel pack sand, due to its high coefficient of friction, presents a relatively "hard" surface to the screen. The wash pipe must be removed prior to production. The disadvantages of this method are similar to those of conventional gravel packing.
In plasticizing or "high viscosity" gravel packing, special fluids are used which are relatively thick containing gelling agents or polymers with the gravel packing particles. A typical fluid will have 15 pounds of material per gallon of water. A string with a gravel pack screen is placed adjacent the producing formation. The high viscosity fluid is pumped into the annulus. Often a technique called "squeezing" is used in which pressure is applied to the emplaced packing in an effort to force packing into the perforations in the casing and cement. There are serious problems with this method. Once the packing operation is completed the gelling agent or polymer should be removed prior to production. Such removal is a tedious operation requiring the introduction of break-out or degradation elements to contact the polymer or gelling agent to release it and reduce its viscosity so it can be pumped out of the wellbore. Also the thickness of the material can cause uneven, non-homogeneous packing with unwanted "bridging" occurring in the packing which prevents homogeneous packing below the bridge.
An unwanted result of adding plasticizing agents directly to the interstitial sand grains may be a notable reduction in permeability. In addition, proper mixing of the resins for the exact wellbore conditions may be difficult. The primary problem, however, is to attain uniform coverage of each perforation without fluid fingering into permeability stratifications.
The particular gravel pack sand used can remain in the wellbore during production and act as a filter to provide clean production fluids for introduction into the production string.
In accordance with .sctn.1.56 of 37 C.F.R. applicants are aware of the following prior art, copies of which are submitted herewith:
1. N-SITU TECHNOLOGY, Section 3, Baker Sand Control, 1984-1985 Catalog, pp. 23-50, 55-56. PA0 2. Introduction to Oil Well Service and Workover, published by The University of Texas, p. 23, 1971. PA0 3. Workover Well Control, by Adams, pp. 4-11. PA0 4. Halliburton Services Sales and Service Catalog, 1970-1971, pp. 130-136. PA0 5. Casing Hardware and Oilfield Service Tools, Dowell, 1982-1983 Composite Catalog, pp. 2520-2523.